1. Field of the Invention
The present invention relates to a golf ball having a polyurethane cover, more particularly, to a golf ball which comprises a solid core and the polyurethane cover covering the solid core, and which exhibits an excellent durability and coloring-resistance against weather without lowering the productivity of the polyurethane cover.
2. Description of the Related Art
As a conventional golf ball, a wound-core golf ball having a “Balata” cover, or a multi-piece golf ball having an ionomer cover is well-known. The wound-core golf ball having a Balata cover is widely used by high-level golfers and professional golfers, because Balata cover is superior in feel and control. Herein, “feel” is the overall sensation to the golfer when the golf ball is hit, and “control” is to impart the motion such as backspin to the golf ball. However, the golf ball having a Balata cover is expensive due to a complicated manufacturing process and is inferior in durability and cut resistance. On the other hand, the golf ball having an ionomer resin cover is also widely used, because of good durability and flight-performance. However, it has been pointed out that the ionomer resin cover is inferior in feel and control to the Balata cover.
In view of these circumstances, a golf ball having a polyurethane cover has been proposed. It is expected that the golf ball having a polyurethane cover exhibits the feel and control which the Balata cover imparts as well as the durability and flight performance which the ionomer cover does.
The two-component type of the polyurethane cover is formed by curing a polyurethane cover composition comprising an isocyanate group terminated polyurethane prepolymer and a polyamine as a curing agent. In the two-component system, it is difficult to adjust the reactivity between the isocyanate group terminated polyurethane prepolymer and the polyamine, thus resulting in the problems of the productivity and the performance of the golf ball. For example, if the reactivity between the isocyanate group and the amino group is too high, the polyurethane cover composition will have the increased viscosity, and sometimes leads to the gelation. Thus, it is impossible to form the polyurethane cover. While if the reactivity is too low, it is necessary to spend longer time on curing the polyurethane cover composition, resulting in the lower productivity. For example, Japanese Patent No. 2,662,909 has disclosed that the reactivity between the isocyanate group terminated polyurethane prepolymer and the polyamine is adjusted by using a polyamine having a low reactivity. The above polyamine having a low reactivity is conventionally used for curing the aromatic polyisocyanate polyurethane prepolymer, such as 4,4′-diphenylmethanediisocyanate, which has a high reactivity. However, the polyurethane cover containing the aromatic polyisocyanate has low weather-resistance, thus yellowing of the polyurethane cover occurs.
The approach to overcome the yellowing of the polyurethane cover includes the use of the aliphatic or alicyclic polyisocyanate. However, since the cohesive energy of the aliphatic or alicyclic polyisocyanate is lower than that of the aromatic polyisocyanate, the use of the aliphatic or alicyclic polyisocyanate tends to lower the mechanical properties of the obtained polyurethane cover. Thus, if the aromatic polyisocyanate is just replaced with the aliphatic polyisocyanate or alicyclic polyisocyanate, the polyurethane cover with the satisfactory performances is not obtained. Further, as described above, if the polyamine having a low reactivity, which is conventionally used for the aromatic polyisocyanate, is applied for the aliphatic or alicyclic polyisocyanate, the reactivity of the polyamine is too low for the aliphatic or alicyclic polyisocyanate, resulting in the lowered productivity.